Will atmospheric storage tank for water cause microbe problem?

The poor penetrative properties of uv make it unsuitable for this
application unless you are running the water thru a small stream into the
storage tank.
The amount of contamination will depend on the BOD of the water. Things
like Pseudomonas and algae grow quite well in low BOD water. Many
storage tanks are kept in the dark for months at a time and microbial
contamination are minimal (eg. cistern drinking water in rural areas).